Osteoporotic fractures are a large and growing health care problem. The recent finding that alleles of the vitamin D receptor (VDR) gene predict bone mineral density is the first direct molecular demonstration of a genetic contribution to the risk for osteoporosis. Recent investigations have demonstrated the VDR allelic effect on intestinal calcium absorption and fracture risk. These differences in VDR gene alleles thus predict differences in bone turnover and bone mass associated with differences in bone and calcium homostasis. Studies of human subjects, though informative, are not adequate to define the mechanism by which the VDR allelic effect is realized. The proposed studies utilize transgenic mice to address the hypothesis that modest alterations in VDR protein level will affect bone turnover. Transgene expression will be regulated either by osteocalcin gene regulatory sequences, for osteoblast-specific expression, or by the H-2k promoter, for general expression in all adult somatic tissues. The osteocalcin gene-regulated expression system will be developed using reporter constructs, prior to the introduction of the VDR transgene. VDR expression will be elevated by a transgenic human cDNA for the VDR, or reduced by using ribozyme or antisense tools. The transgenic animals will be subjected to physiological stresses designed to elicit a change in bone physiology, such as dietary manipulation, treatment with the active hormonal form of vitamin D, or pregnancy and lactation. Parameters that will be monitored include transcription of vitamin D-responsive genes, serum regulators of calcium homeostasis, and histological and histomorphometric aspects of bone structure. It is anticipated that in animals with elevated bone VDR levels, the whole-body response to physiological stress will be shifted onto the bone, resulting in net bone loss. Once conditions under which physiological differences between transgenic and nontransgenic mice have been defined, it should be possible to identify conditions under which the same mice will be able to maintain normal bone mass. It is the broad aim of this project to use this information to identify potential therapeutic targets in humans, based on an understanding of the molecular and physiological mechanisms of the VDR allelic effect.